This invention relates to a linearity-adjusting circuit for adjusting, usually correcting, the rate of change of amplitude of an output signal with respect to the amplitude of an input signal from which the output signal is derived. In most applications the output signal amplitude is nominally proportional to the input signal amplitude. The invention is particularly useful, for example, in a high frequency pre-distortion circuit for signals which are to undergo amplification in power amplifiers, such as klystrons which are used in television transmission. Klystrons, and indeed many high power amplifiers and the like, introduce undesirable signal distortions, and a linearity-correcting circuit is required for ensuring a linear amplitude relationship between the envelope of the modulated and power-amplified transmission signal and the video input signal which was used in the initial modulation process.
In some existing colour television transmitter chains, an appropriate pre-distortion is introduced into the intermediate frequency signal before the waveband-dependent radio frequency stage, for linearity correction. To obtain the correct amount of pre-distortion over the full amplitude range, variable impedances or other elements of the pre-distortion circuit are adjusted manually until the envelope of the transmitted signal has a sufficiently linear relationship with the envelope of the input modulated signal, over the entire range of envelope amplitudes. Since the characteristics of the transmitter chain vary in operation with supply voltage levels and ambient temperature amongst other things, it is found that re-adjustment of the circuit elements is necessary quite frequently. If the non-linearity occurs only over a narrow band of amplitudes, it is found that adjustment of just one circuit element may be sufficient to correct this particular non-linearity, but that it then introduces non-linearities at other amplitude levels. Consequently, even for a slight correction, it is necessary to adjust more than one of the circuit elements. Moreover, because of the interdependence of the settings of the circuit elements, they frequently require more than one adjustment to compensate for each change in power-amplifier characteristics; considerable skill and experience is required in order to judge the degree of adjustment required.